Tumor Cell-derived HMGB1-initiated Activation of Treg Abstract High-mobility group box 1 (HMGB1) in tumor cells and the tumor microenvironment (TME) is tied to the hallmarks of tumor development and progression, but its role in tumor-induced immune suppression is unclear. We found that stable silencing of HMGB1 in tumor cells or specific inhibiting of extracellular HMGB1 in the TME reduced tumor-induced activation of regulatory T cells (Treg) and uncovered spontaneous CD8 T cell-dependent antitumor immunity in multiple murine tumor models, suggesting that tumor cell-derived HMGB1 may trigger Treg activation. Also, we found that tumor cell-derived HMGB1 promoted the production of thymic stromal lymphopoietin (TSLP) by tumor cells. Further, we found that tumor cell-derived HMGB1 and TSLP acted together to modulate dendritic cells (DC) to activate Treg. We hypothesize that growing tumor cell-derived HMGB1 acts as an extracellular signal on tumor cells to heighten TSLP, which in turn `partners' with HMGB1, to modulate DC to activate Treg. We will test this central hypothesis with 2 specific aims: 1) Define how HMGB1 heightens TSLP in tumor cells 2) Dissect how HMGB1 modulates DC to activate Treg This study will have led to a novel concept that HMGB1 is immune suppressive during tumor progression. Novel findings -- that uncover a previously unknown function of HMGB1 as an extracellular signal elevating TSLP in tumor cells and reveal an innovative mechanistic connection between HMGB1 and TSLP in Treg activation via DC -- will advance our understanding of how tumors induce Treg activation.